1. Field of the Invention
The present invention relates, in general, to a method for the fabrication of a semiconductor device and, more particularly, to a method for forming a capacitor in a DRAM, capable of relieving the step between the cell region and the peripheral circuit region, thereby securing a sufficient allowance of depth of focus subsequent lithography processes in addition to improving production yield and reliability.
2. Description of the Prior Art
In accordance with the recent trend of high integration in semiconductor devices, in becomes more difficult to form a capacitor with sufficient capacitance as the size of the cell decreases. In order to avoid this difficulty, there has been suggested many solutions. For example, a dielectric with a high dielectric constant or a thin dielectric film is employed in a DRAM device consisting of a MOS transistor and a capacitor, with the aim of increasing the capacitance in the capacitor.
However, these solutions have their own problems. For example, Ta.sub.2 O.sub.5, TiO.sub.2 and SrTiO.sub.3, known as dielectrics with a high dielectric constant, have been intensively studied. Nonetheless, these materials are virtually impossible to apply in practice because their reliability, such as junction breakdown voltage, and thin film characteristics are not yet verified. The thinning of thickness of dielectric film is likely to break the dielectric film during the operation of a device, thereby adversely affecting the capacitor.
Apart from this solution for the dielectric, an increase in the surface area of a storage electrode has been suggested with respect to the difficulty in securing sufficient capacitance. For example, in order to increase the surface area of a capacitor, a pin structure in which a plurality of polysilicon layers are connected with one another by a conductive material going through them, or a cylindrical structure is adopted.
In order to better understand the background of the invention, an illustrative description for a conventional semiconductor device adopting a cylindrical capacitor will be given below.
Following establishment of a MOSFET on a semiconductor substrate, an interlayer insulating film is formed on the entire surface of the resulting structure. A bit line is formed in contact with the drain of the MOSFET. Thereafter, a blanket planarization layer with good step coverage, for example, BPSG (boro phospho silicate glass), is furnished upon the resulting structure and then, subjected to etch, to form a contact hole for a storage electrode through which the source of the MOSFET is exposed. A conductive layer is given to bury the contact hole for the storage electrode. Subsequently, a cylindrical, thick oxide film pattern is formed over the contact hole and a cylindrical conductive spacer is formed at the side wall of the oxide pattern. With a mask of both the oxide pattern and the conductive spacer, the conductive layer is etched, so as to leave a conductive layer pattern which still fills the contact hole. Then, the cylindrical, thick oxide film pattern is removed, to complete the formation of a cylindrical storage electrode consisting of the conductive spacer and the conductive layer pattern.
As mentioned above, high integration of a semiconductor device requires that the area occupied by a capacitor be diminished in a semiconductor device. This directs the semiconductor device to the problem of decreasing the capacitance. One conventional solution to the problem is to heighten the storage electrode. However, the towering of a storage electrode brings about higher augmentation of the step between the cell region and the peripheral circuit region, thereby bringing the subsequent processes into difficulties. For example, a lithography process for the formation of metal wiring has difficulty in forming a precise pattern. Accordingly, there is a significant inferiority in production yield and reliability of device operation.
Since the above-illustrated conventional technique uses contact mask and a storage electrode mask separately, the processing becomes complicated and the production yield decreased.